<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .dropdown-btn {
            position: relative;
            width: 100px;
            height: 50px;
            border: 2px solid #ddd;
            box-sizing: border-box;
            text-align: center;
            line-height: 50px;
            margin: 0 auto;
            border-bottom: none;
            background: white;
        }
        .dropdown-btn:hover .dropdown-menu{
            display: block;
        }
        .dropdown-menu {
            position: absolute;
            top: 46px;
            left: -2px;
            width: 300px;
            height: 200px;
            border: 2px solid #ddd;
            box-sizing: border-box;
            display: none;
        }
        .line {
            width: 996px;
            height: 2px;
            background: #fff;
            position: absolute;
            top: 46px;
            left: 0;
            z-index: 1;
            display: none;
        }
    </style>
</head>
<body>
    <div class="dropdown-btn">
        下拉按钮
        <div class="line"></div>
        <Div class=dropdown-menu>
            这是下拉菜单
        </Div>
    </div>
</body>
</html>